Apparatus for measuring dielectric constant

ABSTRACT

A very high frequency oscillator has an antenna as the frequency determining element. The antenna is immersed in a material having a high ionic conductivity, and the shift in the frequency of the oscillator is measured. The frequency shift is indicative of the dielectric constant and moisture content of the material. A dielectric may be interposed between the antenna and the material to reduce the range of frequency shift of the oscillator.

United States Patent Lundstrom APPARATUS FOR MEASURING DIELECTRICCONSTANT Inventor: John W. Lundstrom, Glendora,

Calif.

bra, Calif.

Filed: April 7, 1970 Appl. No.: 26,244

US. Cl. ..324/6l QS, 324/585 A Int. Cl. ..G0lr 27/26, GOlr 27/04 Fieldof Search 324/585, 58.5 A,

58.5 B, 58.5 C, 324/61 TI [56] References Cited UNITED STATES PATENTS3,550,106 12/1970 Gehman ..33l/6 5X Assignee: Moisture Register Company,Alham- Oscillotor Antenna Frequency Meosur i ng Apparatus 1 Aug. 15,1972 FOREIGN PATENTS OR APPLICATIONS 991,795 5/ 1965 Great Britain"324/585 l,082,872 9/1967 Great Britain ..324/6l Tl 349,102 9/1960Switzerland ..324/6l TI Primary Examiner-Alfred E. SmithAttorney-Forrest J. Lilly ABSTRACT 2 Claims, 3 Drawing Figures MaterialTo Be Measured PAlENTElllus l 5 I972 3.684.952

Fig l.

Material Oscillator Antenna To Be Measured l l l I3 Frequency MeasuringApparalus John W. Lundsrrom,

INVENTOR.

ATTORNE APPARATUS FOR WASURING DIELEC l CONSTANT BACKGROUND OF THEINVENTION of moisture content in materials is the determination of ldielectric constant. These dielectric constant measurements generallyare made at audio frequencies or medium to high radio frequencies, andeither employ a balanced bridge or a frequency shifted oscillator as abasic measurement circuit.

There exists a large class of materials containing high moisturecontents and varying amounts of substances which create a highdielectric power loss in these materials. In fact, many materialsexhibit such a high conductance to the measuring circuit resulting fromhigh material temperature, high ionic conductivity and/or high moisturecontent, so as to render impossible the measurement of dielectricconstant by conventional means.

Materials exhibiting high ionic conductivity at lower frequencies tendto exhibit a reduced ionic conductivity as the frequency is increased.However, as the measurement frequency is increased, conventionalmeasuring circuits become less useful. A bridge type of measuringcircuit becomes extremely difficult to balance because stray capacitiesare so evident. Use of the usual frequency shifted oscillator techniquesrequire rather small samples to keep the capacitance value of the sampleloaded capacitor small, to afford proper resonant circuits at higherfrequencies. Coupling of the measuring oscillator to the sample becomesquite difiicult due to inductances created in the lead lengths required.Transmission line techniques are not particularly useful in coupling tothe electrode due to the change in oscillator frequency. Also, in theapplication of these techniques, the remaining component of loss tendsto reduce the resonant tank circuit Q to values which often render thetank circuit ineffective in determining oscillator frequency, and straycapacitance and lead lengths form higher Q tank circuits resulting inoscillation at other than desired frequencies.

OBJECTS OF THE INVENTION Accordingly, it is an object of the inventionto provide a novel method of measuring dielectric constant and moisturecontent of materials having a high dielectric power loss due to highionic conductivity.

Another object of the invention is the provision of novel apparatus formeasuring dielectric constant and moisture content of materials having ahigh dielectric power loss due to high ionic conductivity.

Yet another object of the invention is to provide apparatus formeasuring dielectric constant which operates in the very high frequencyrange.

SUMMARY OF THE INVENTION In accordance with these and other objects ofthe invention, there is provided a very high frequency oscillatoremploying an antenna as the frequency determining element or tankcircuit. The antenna is immersed in the material to be measured causinga shift in the oscillator frequency. This frequency shift is measured,and is indicative of the dielectric constant and moisture content of thematerial.

BRIEF DESCRIPTION OF THE DRAWINGS The following specification and theaccompanying drawings describe and illustrate exemplary embodi- 0 mentsof the present invention. Consideration of the specification and thedrawings will provide an understanding of the invention, including thenovel features and objects thereof. Like reference characters denotelike parts throughout the figures of the drawings.

FIG. 1 is a diagram of an embodiment of apparatus for measuringdielectric constant in accordance with the invention;

FIG. 2 is a perspective view of one form of antenna arrangement for theapparatus of FIG. 1;

FIG. 3 is a diagram of another form of antenna arrangement andassociated oscillator circuit for the apparatus of FIG. 1 and FIG. 4 isa pictorial, perspective view of the antenna arrangement of FIG. 3incorporating a conducting cavity.

DESCRIPTION OF THE INVENTION Referring now to FIG. I, there isillustrated apparatus for measuring dielectric constant constructed inaccordance with the present invention. An oscillator 10 is coupled to anantenna 11 which serves as the frequency-determining element or tankcircuit thereof. The parameters of the oscillator 10 and antenna 11 areselected or adjusted so that the oscillator 10 oscillates at a frequencyin the VHF region, that is, a frequency between and 300 MHZ. Thematerial 12 whose dielectric constant is to be measured is coupled tothe antenna 11 and causes the frequency of the oscillator 10 to change.Frequency measuring apparatus 13 is coupled to the oscillator 10 formeasuring changes in the frequency thereof, and thereby indicate thedielectric constant of the material 12. A piece of dielectric material14 may be interposed between the antenna 11 and the material 12 to bemeasured to vary the sensitivity of frequency shift versus dielectricconstant changes.

The antenna 11 may be any one of several different types, for example, ahalfwave dipole antenna. The resonant frequency of a halfwave dipoleantenna is inversely proportional to the square root of the relativedielectric constant of the medium in which the antenna is immersed.Thus, a halfwave dipole antenna of fixed length exhibits a reduction ofresonant frequency as the relative dielectric constant of the media inwhich it is immersed is increased. Since the antenna 11 is employed asthe frequency determining element or tank circuit of the oscillator 10,the frequency of the oscillator I0 is inversely proportional to thesquare root of the relative dielectric constant of the substance inwhich it is immersed.

FIG. 2 shows the antenna 11 placed coaxially in a cylindrical container15 having a diameter of at least a half wavelength. The material 12 tobe measured is placed in the container 15 and surrounds the antenna 11.The material 12 to be measured may be any wet material having a highionic conductivity such as wetspent grain or bauxite ore. The shift infrequency of the oscillator when the'material 12 is placed in the container is indicative of the relative dielectric constant of the material12. Furthermore, the shift in frequency of the oscillator 10 is .alsoindicative of the moisture content of the material 12. Water has arelative dielectric constant of about 80 and when it is added tomaterials with relative dielectric constants of 5 to 10 the water causesa substantial increase in the dielectric constant of the base material.

Because the measurement is made in the VHF frequency range, thedielectric power loss, high conductance, high ionic conductivity or highmoisture content do not render measurement impossible as happens whenthe measurement is made by conventional means. Materials exhibiting highionic conductivity at lower frequencies tend to exhibit a reduced ionicconductivity as frequency is increased. Several orders of magnitude inthe reduction of dielectric power loss is often obtained by employingmeasuring frequencies in the VHF region. Apparently the transfer of ionscannot take place at these frequencies at a rate to substantiallyincrease the dielectric power loss. Furthermore, temperature variationsin the dielectric power loss factor resulting from high ionicconductivity occurring at lower frequencies are substantially reduced bymaking the measurement in the VHF frequency region.

The frequency measuring apparatus 13 for determining the frequency shiftof the oscillator 10 may take any one of several different forms. Thefrequency measuring apparatus 13 may be a VHF receiver tuned to thefrequency of the oscillator 10 before and after loading the antenna 11with the material 12 to be measured. The frequency measuring apparatus13 may also be apparatus for making a direct frequency measurementemploying frequency division and pulse counting, or it may be frequencycomparison apparatus which compares the oscillator 10 to a fixed localoscillator.

By using the antenna 11 as the frequency determining element of theoscillator 10, the problem of trying to balance a bridge type instrumentat VHF frequencies is eliminated. Furthermore, the problems of capacitorsize, lead length, and stray capacitance and inductance are eliminated.The size of the sample of the material 12 to be measured can be larger,and is approximately a half wavelength long and a quarter to a third ofa wavelength in diameter. Relatively large frequency shifts arepractical with this apparatus, rendering the stability considerations ofthe measuring circuitry less critical.

When measuring materials with a high power loss, it may be found thatthe half wave dipole is not entirely satisfactory. The feed pointimpedance of a halfwave dipole in free space is approximately 72 ohms.In a lossy dielectric medium the feed point impedance is reduced as afunction of the loss factor and dielectric constant. The negativeresistance of the active element of the oscillator 10 must besubstantially lower than the load provided by the antenna 11. This maybe difficult to achieve with conventional oscillating devices whenmeasuring materials with a high power loss.

This problem may be overcome in many cases by using a folded dipoleantenna, whose impedance is about 300 ohms in free space. Hence, theimpedance presented to the terminals of the oscillating device is aboutfour times that of a halfwave dipole under the same material loadingconditions.

FIG. 3 shows a slot type antenna 11 which is even better suited tomeasuring materials with a high power loss. The slot antenna 11 isdisposed in a conducting plane 16, and the slot is a half wavelengthlong and onetwentieth to one one-hundreth of a wavelength wide. Thedriving impedance at the midpoint of the slot is about 363 ohms andapproximately doubles when one side of the conducting plane is enclosedin a conducting cavity to prevent radiation from the slot on that sideof the plane. A pictorial view of a slot antenna with one side of theconducting plane 18 enclosed in a conducting cavity 19, as shown in FIG.4. A slot antenna radiating from one side of a plane sheet will presenta free space impedance of about 700 to 900 ohms. This value of load iswell adapted for us with conventional two terminal oscillating devicesexhibiting negative impedance at the load temiinals, especially whenloaded with materials of high loss factor.

The oscillator 10 may be any active device exhibiting sufficiently lownegative resistance and small reactive components at its two loadterminals. The tunnel diode oscillator is particularly well suited foruse with the slot antenna, and FIG. 3 illustrates a tunnel diode l7 andits associated circuitry (which is conventional) coupled to the slotantenna 1 1. The oscillation frequency is dependent on the internalcapacitance of the 17, the dimensions of the slot antenna 11, and thedielectric material 12 in proximity to the radiating face of the slotantenna 11. The size of the sample of the material 12 to be measureddepends on the penetration of the radiated energy, and is approximatelya half wavelength long and a quarter wavelength in all directionsperpendicular to.

the slot axis. Wavelength in all cases is the wavelength either in freespace or in the dielectric material 12 loading the antenna 11. Thewavelength is directly related to the slot dimensions since frequency isthe variable with material dielectric constant changes.

To reduce the frequency shift of the oscillator 10 to be within therange of the frequency measuring apparatus l3, pieces of dielectricmaterial 14 may be in serted between the material 12 to be measured andthe antenna 11. For slot lengths of about 12 inches and oscillatingfrequencies of about 170 MHZ, it will be found that pieces of fiberglassimpregnated with epoxy resin, or Teflon, from one-sixteenth to one-halfinch thick will be satisfactory for reducing the frequency shift.

Apparatus in accordance with the present invention was constructed tooperate at a frequency in air of approximately l MHZ and the dielectricconstant, and therefore moisture content, of wet-spent grain wasmeasured. It was found that when the moisture content by wet weight ofthe wet-spent grain was varied from approximately 25 percent toapproximately 50 percent, the frequency shifted approximately 25 MHZ.

Although several variations in the practice of the invention have beenshown and described, other variations may be made and it is intendedthatthe foregoing disclosure shall be considered only as an illustration ofthe principles of the invention and not construed in a limiting sense.

What is claimed is:

1. Apparatus for providing an indication of moisture content of materialhaving a high ionic conductivity comprising:

a conducting plane having a slot therein forming a very high frequencyantenna, one side of said conducting plane being enclosed in aconducting cavity to prevent radiation from said slot on that side ofsaid conducting plane;

oscillating means having a low negative resistance coupled to said slotin said conducting plane, with said slot forming the frequencydetermining element for said oscillating means, said oscillating meansoscillating in the range between 100 and 300 megahertz;

frequency measuring means coupled to said oscillating means forindicating shifts in frequency thereof;

said conducting plane being adapted for immersion of the slot therein inmaterial having a high ionic conductivity for causing a shift in thefrequency of said oscillating means indicative of the moisture contentof the material.

2. The method of measuring the moisture content of material having ahigh ionic conductivity with an oscillator having an antenna as thefrequency determining element thereof, the method comprising thefollowing steps:

adjusting the oscillator to oscillate in the range of between and 300megahertz;

measuring the frequency of the oscillator with the antenna in air;

immersing the antenna in material having a high ionic conductivity;

measuring the frequency of the oscillator with the antenna immersed inthe material; and determining the frequency shift between the twofrequency measurements.

1. Apparatus for providing an indication of moisture content of materialhaving a high ionic conductivity comprising: a conducting plane having aslot therein forming a very high frequency antenna, one side of saidconducting plane being enclosed in a conducting cavity to preventradiation from said slot on that side of said conducting plane;oscillating means having a low negative resistance coupled to said slotin said conducting plane, with said slot forming the frequencydetermining element for said oscillating means, said oscillating meansoscillating in the range between 100 and 300 megahertz; frequencymeasuring means coupled to said oscillating means for indicating shiftsin frequency thereof; said conducting plane being adapted for immersionof the slot therein in material having a high ionic conductivity forcausing a shift in the frequency of said oscillating means indicative ofthe moisture content of the material.
 2. The method of measuring themoisture content of material having a high ionic conductivity with anoscillator having an antenna as the frequency determining elementthereof, the method comprising the following steps: adjusting theoscillator to oscillate in the range of between 100 and 300 megahertz;measuring the frequency of the oscillator with the antenna in air;immersing the antenna in material having a high ionic conductivity;measuring the frequency of the oscillator with the antenna immersed inthe material; and determining the frequency shift between the twofrequency measurements.